Corrosion inhibitors in polyurea coatings

ABSTRACT

Durable elastomeric polyurea coatings can be applied to metal surfaces subjected to impacts and environmental corrosion. Corrosion resistance of the coatings, especially on magnesium alloy surfaces, is improved by incorporation of particulate inhibitor additives in the liquid polyurea precursor coating material. Examples of inhibitors include potassium dichromate, sodium dichromate, potassium permanganate, vanadium pentoxide, phosphorus anhydride, cerium oxide, lanthanum oxide, calcium oxide and sodium oxide.

TECHNICAL FIELD

This invention pertains to corrosion resistant polymer coatings for metal surfaces. More specifically, this invention pertains to the use of corrosion inhibitors in elastomeric polyurea coatings for magnesium alloy surfaces.

BACKGROUND OF THE INVENTION

Light weight cast and wrought magnesium alloys have been developed for automotive applications. In some such applications they are exposed to conditions in which they undergo galvanic corrosion. For example, contact of the magnesium part with water and salt and/or with dissimilar metals leads to the corrosion of the affected magnesium alloy surface. Many automotive chassis and body components require protection from galvanic corrosion.

Polymer coatings have been used on metallic parts to provide scratch-resistance, chip-resistance and resistance to corrosion. Epoxy resins and polyurea resins have been developed for individual usage and in combination for these applications. Commercially available polyurea resins in precursor formulations can be applied to, for example, a truck bed by spraying or dip-coating, and rapidly cured. Also, epoxy resins in sprayable powder form are available as metal surface coatings. But sprayable epoxy powder resins are quite expensive, and polyurea resins have not provided sufficient corrosion resistance.

It is an object of this invention to provide elastomeric polyurea resin formulations with corrosion inhibitors for metal surfaces and especially magnesium alloy surfaces.

SUMMARY OF THE INVENTION

In the context of this invention, the term “polyurea” refers to polymerization products of the reaction of a suitable poly-functional isocyanate monomer(s) with a suitable polyfunctional amine monomer(s). The polyfunctional monomers provide many urea links [—NH—(CO)—NH—] between the isocyanate and amine moieties of the polymer chains, and between polymer chains. Such materials are known and commercially available. They typically combine impact resistance, heat resistance up to 150° C., and insensitivity to moisture, all desirable properties for components of automotive vehicles and the like. A mixture of monomers or prepolymers is formulated for spray coating or immersion coating onto a metal substrate such as a magnesium alloy casting or formed panel. Catalysts and chain extenders may be incorporated to provide for a suitable cure rate of the primary and secondary amine precursors and isocyanate precursors, or prepolymer precursors, to allow leveling of the sprayed coating and adhesion to the cleaned workpiece surface before the cure is completed. Coextensive polyurea coatings applied on metal surface provide effective and durable protection for the underlying metal. However, truck beds and the like are subjected to severe wear and tear and the coatings are chipped or scratched to expose the underlying metal to corrosive media such as salt water. Exposed magnesium alloy body components are susceptible to corrosion in these circumstances.

In accordance with this invention, particulate corrosion inhibitors are mixed with the coatable polyurea precursor formulation. The inhibitor particles are used in sufficient abundance in the matrix of the cured elastomer to inhibit corrosion of an underlying magnesium alloy surface. The corrosion inhibitors can be identified and selected by suitable testing procedures with a magnesium alloy selected for a particular application. In general metal oxides and metal oxide containing compounds are suitable corrosion inhibitors. Examples of inhibitors to impede electrolytic corrosion include potassium dichromate, sodium dichromate, potassium permanganate, vanadium pentoxide, and phosphorus anhydride. Examples of inhibitors that provide barrier-film type corrosion protection when incorporated in the polyurea include cerium oxide and lanthanum oxide. And examples of inhibitors for inclusion in a polyurea that help maintain an alkaline environment for magnesium passivation include calcium oxide and sodium oxide. These corrosion inhibiting additives may be used alone or in combinations in the polyurea for protection of different magnesium alloys used in different parts subjected to different corrosion inducing environments. Inhibitor additions in the amount of about one-tenth weight percent to about one weight percent of the polymer coating material are usually suitable. The amount of inhibitor addition is a balance between obtaining useful corrosion inhibition without compromising the bonding strength of the coating to the underlying metal or polymer surface. The particle sizes of the inhibitors are suitably in the range of about five to about fifty micrometers or as necessary to obtain a desired smoothness of the polyurea coating.

The mixture of inhibitor particles and polyurea precursors are suitably applied to the surface of a magnesium alloy workpiece and cured to a durable scratch and corrosion resistant finish. Typical coating thicknesses are in the range of about one-half millimeter to about two millimeters. The material may be applied as a primary coating on the magnesium surface, or over an earlier applied coating, such as a conversion coating or an epoxy powder coating.

Other objects and advantages of the invention will be come apparent from a description of preferred embodiments.

DESCRIPTION OF PREFERRED EMBODIMENTS

The inhibitor-containing polyurea coating compositions of this invention can provide beneficial protection to many different metal workpieces but the practice of the invention is particularly applicable to articles of manufacture for automotive vehicles based on magnesium alloys because of their tendency to corrode in their environment.

Commercial magnesium alloy systems adapted for sand and permanent mold castings include magnesium-aluminum-manganese (AM), magnesium-aluminum-zinc (AZ), magnesium-rare earth-zirconium (EK, EZ, and ZE), magnesium-zinc-zirconium (ZK), and magnesium-thorium-zirconium (HK, HZ, and ZH). AZ and AM alloys are also used in high-pressure die casting applications. Other die cast alloys include magnesium-aluminum-silicon (AS) magnesium-aluminum-strontium (AJ), magnesium-aluminum-rare earth (AE). AM50 is a representative magnesium-based casting alloy of typical nominal composition, by weight, of 5% aluminum, 0.4% manganese, and the balance magnesium. Compositional specifications, temper specifications, and physical properties for alloy members of these systems are available from commercial sources and technical references. Wrought magnesium alloys, produced as bars, billets, shapes, wire, sheet, plate, and forgings, are often made using members of the AZ system, such as AZ31B,C having a typical nominal composition, by weight, of 3.0% aluminum, 0.3% manganese, 1% zinc, and the balance magnesium. Extruded bars, rods, tubes, and the like, may be made of magnesium alloy systems such as AZ80A and ZK60A. The practice of the invention is applicable to the families of magnesium alloys.

Spray polyurea coatings are applied to metal surfaces (ferrous alloys, aluminum alloys, and magnesium alloys) on commercial and personal vehicles. Typical coating applications include truck bedliners, quarter panels, cab floor liners, and trailer liners. Prepolymer precursors of the coatings are formulated by commercial suppliers for spray application or coating by immersion of the parts in a bath. The original monomeric constituents often include aromatic diisocyanates (and higher functionality isocyanates) and suitable primary and secondary diamines and triamines. Mixtures of the 4, 4 ′ isomer and 2, 4′ isomer of methylene bisphenyl isocyanate (MDI) may be used to adjust curing rate. In the case of polyurea spray coatings, a liquid volume containing the diisocyanate is prepared separately from the polyamine-containing volume and the two streams are mixed in suitable proportion in a high pressure spray gun for application on a workpiece surface. In accordance with this invention, one or more corrosion inhibitors are incorporated into the liquid prepolymer formulation(s), especially for coatings on magnesium alloy workpieces.

Examples of inhibitors to impede electrolytic corrosion include potassium dichromate, sodium dichromate, potassium permanganate, vanadium pentoxide, and phosphorus anhydride. Examples of inhibitors that provide barrier-film type corrosion protection when incorporated in the polyurea include cerium oxide and lanthanum oxide. And examples of inhibitors for inclusion in a polyurea that help maintain an alkaline environment for magnesium passivation include calcium oxide and sodium oxide. The selection of one or more inhibitors for a particular cast or formed magnesium alloy is assisted by preliminary testing of a candidate inhibitor with the specific alloy. Polyurea coatings applied on metal surfaces like pickup truck beds provide an effective and durable protection for the underlying metal. However, once the coating is damaged, by scraping, scribing or the like, exposed metal (especially a magnesium alloy) is susceptible to corrosion. Once exposed to salt water or other corrosive media, inhibitor particles included in the polyurea coating release species (e.g., ions) to interact with the metal to form a stable barrier layer, a passive film, or an alkaline environment to mitigate corrosion.

Automotive vehicles are frequently exposed in normal usage to aqueous salt solutions, and immersion of a candidate magnesium alloy casting or panel in such a solution provides a test for selection of an inhibitor material with respect to this type of corrosion. For example, a cold rolled panel of AM50 alloy was immersed in a five weight percent aqueous sodium chloride solution at room temperature in a vessel in which the rate of evolution of hydrogen gas could be measured. In the absence of any inhibitor the solution did slowly react with the panel and a rate of hydrogen release of about 31 liters per hour per square centimeter of panel surface (L/hr/cm²) was measured. This hydrogen release rate was used as a control value for comparison with the effect of candidate corrosion inhibitors by this corrosion mechanism.

Four additional solutions, each of five weight percent aqueous sodium chloride were prepared containing, respectively, one gram per liter (1 g/L) CrO₃, 1 g/L La₂O₃, 1 g/L Ca(OH)₂, and 1 g/L K₂Cr₂O₇. An AM 50 test panel was immersed in each of the four solutions and the corresponding hydrogen release rates measured. The hydrogen release rates for the candidate inhibitor additives were as follows: CrO₃, 45 L/hr/cm²; La₂O₃, 20 L/hr/cm²; Ca(OH)₂, 6 L/hr/cm²; and K₂Cr₂O₇, 5 L/hr/cm². In these tests the CrO₃ was ineffective in inhibiting corrosive attack by aqueous sodium chloride on the AM 50 panel. But La₂O₃, Ca(OH)₂ and K₂Cr₂O₇ did inhibit corrosive salt attach on the magnesium alloy in salt solution.

Following the above aqueous sodium chloride tests on bare AM50 panels, three batches of a commercial polyurea spray formulation were prepared to contain, respectively one weight percent each of, La₂O₃, Ca(OH)₂ and K₂Cr₂O₇ as inhibitor materials. AM50 panels were spray coated to a thickness of about one to two millimeters with inhibitor additive-containing polyurea.

Two Huntsman Corporation polyurea precursor liquids at about 160° F. were combined for spraying in a Gusmer high pressure gun-type, spray machine operating at about 2000 psi. The combined polyurea precursor streams were sprayed onto the panel surfaces to a thickness of about one to two millimeters. One precursor liquid volume contained the MDI mixture (Suprasec 9520) and the second precursor mixture contained amine terminated polyols, chain extenders and one of the inhibitor additives identified above. The spray polyurea elastomer coatings quickly gelled on the surfaces of the panels and were dry within about ten seconds.

Some of the coated panels were then scribed through the coating to expose the metal surface to initiate a situation of paint damage. Coated and scribed panels were than tested in a cyclic corrosion chamber using a standard test procedure, GM 9540P. This accelerated corrosion test comprises three eight hour steps (or shifts) per day which may be repeated over many days. In the first eight hour shift (called Dry Soak), the coated and scribed panels are exposed to an air atmosphere at 30% relative humidity at 140° F. In the second shift, the panels at ambient temperature are subjected to four successive 1.25% salt solution mist sprays. The successive sprays are separated by at least one hour in the eight hour shift. In the third eight hour shift, the salt mist-coated panels are held in air at 100% relative humidity (Wet Soak) at about 120° F. This 24-hour process constitutes one test cycle. This testing of the scribed coated panels was repeated over thirty days. No observable corrosion occurred in any of the scribed samples after 30 days (4 years field equivalent).

While chromates and dichromates are useful in many polyurea coating applications to provide corrosion resistance to magnesium alloy components it is recognized that there may be some release of chromium to the environment. For this reason it may be preferred to use the alkaline metal oxide inhibitors, such as the oxides of calcium, sodium, lanthanum or cerium, where these oxide inhibitors or their equivalents provide suitable inhibition of corrosion.

The practice of the invention has been illustrated by examples of preferred embodiments, but the scope of the invention in not limited to the specific examples. 

1. A coating material for a magnesium alloy surface of an article, the coating material comprising: liquid precursors of an elastomeric polyurea formulated for spray coating or immersion coating of the magnesium alloy surface, the precursors containing particles of a corrosion inhibitor for the magnesium alloy.
 2. A coating material for a magnesium alloy surface as recited in claim 1 in which the corrosion inhibitor is at least one of potassium dichromate, sodium dichromate, potassium permanganate, vanadium pentoxide, and phosphorus anhydride.
 3. A coating material for a magnesium alloy surface as recited in claim 1 in which the corrosion inhibitor is at least one of cerium oxide and lanthanum oxide.
 4. A coating material for a magnesium alloy surface as recited in claim 1 in which the corrosion inhibitor is at least one of calcium oxide and sodium oxide.
 5. A coating material for a magnesium alloy surface as recited in claim 1 in which the polyurea comprises the 4, 4′ isomer and 2, 4′ isomer of methylene bisphenyl isocyanate.
 6. A component for an automotive vehicle, the component comprising a magnesium alloy surface with a coating of an elastomeric polyurea, the elastomeric polyurea comprising particles of a corrosion inhibitor for the magnesium alloy in an amount up to about one percent by weight of the polyurea.
 7. A component for an automotive vehicle as recited in claim 6 in which the corrosion inhibitor is at least one of potassium dichromate, sodium dichromate, potassium permanganate, vanadium pentoxide, and phosphorus anhydride.
 8. A component for an automotive vehicle as recited in claim 6 in which the corrosion inhibitor is at least one of cerium oxide and lanthanum oxide.
 9. A component for an automotive vehicle in which the corrosion inhibitor is at least one of calcium oxide and sodium oxide.
 10. A component for an automotive vehicle in which the polyurea comprises the 4, 4′ isomer and 2, 4′ isomer of methylene bisphenyl isocyanate. 